This invention relates to a telephone subscriber line circuit and, more particularly, to a battery feed circuit of small size and low power consumption for a digital telephone electronic switching system.
A battery feed circuit is used in a telephone switching system to supply direct current from a battery power supply to a two-wire subscriber loop which transmits voice signals to and from a telephone transmitter/receiver.
In a digital telephone switching system, as shown in FIG. 1, a subscriber line circuit is located between the subscriber line and the switching network. The subscriber line circuit has "BORSCHT" functions (Battery power feed to subscriber line, Overvoltage protection from subscriber line, Ringing to telephone set, Supervision of status of line loop, Coder and decoder of telephone signal, Hybrid circuit to convert between a two-wire and a four-wire system, and Testing) and a battery feed circuit, and is provided for each subscriber line. This includes the circuits for the "BORSCHT" functions as indicated by the components labelled with the respective letters B.sub.1, O, R, S, C, H and T in FIG. 1.
The total number of battery feed circuits needed in a system is equal to the number of the subscriber lines. In a large capacity digital system which accomodates many subscriber lines, therefore, a battery feed circuit of small size and low power consumption is essential for designing compact subscriber line equipment.
As the battery feed circuit of low power consumption, the constant current feed circuit and the floating battery feed circuit are well known. However, the former circuit has the problems of producing excessive heat loss in the circuit and of losing the automatic gain control (AGC) function of a telephone transmitter circuit for a telephone set.
The floating battery feed circuit has the problems of being bulky because a large transformer is used in the power supply circuit and of aggravating the problem with the AGC function of a telephone circuit, because the power feed characteristics are different from the characteristics of a constant resistance power feed.
A constant resistance power feed has been used since the availability of an early type of analog system, and is preferrable for the AGC function of a telephone set and for protection of the power feed circuit from large currents in the case of an accident on the subscriber line.
There are two types of known circuits of constant resistance power feed, namely a transformer circuit and an electronic circuit.
A transformer circuit of the prior art is shown in FIG. 2. The circuit includes a large transformer T with two windings. The first winding is connected between a first wire A of a subscriber line and a negative terminal V.sub.B of a battery, and the other winding is connected between a second wire B and the positive-grounded terminal G of the battery.
It is required for the battery power feed circuit to have a high impedance with respect to an alternating current (AC) telephone signal and a constant impedance with respect to direct current (DC) voltage. Low impedance for an AC induction noise signal is also required.
In a transformer circuit, a high impedance to the AC telephone signal is easily realized by providing the inductance L with a large value, because the AC impedance is given by 2.pi.fL, wherein f is the frequency of the AC signal. The DC impedance of the transformer is the resistance of the windings. Therefore, it is easy to obtain the desired resistance of the transformer. A small value of the impedance for the induction noise is realized by choosing the direction of the two windings to be such that the magnetic flux of each winding cancels that of the other. The AC impedance for the induction noise, looking from wires A and B towards the transformer, therefore is equal to the sum of the DC resistances of the two windings.
Thus, the transformer circuit fulfills all of the requirements for the power feed circuit. However, miniaturization of the transformer is a problem.
An example of the electronic type power feed circuit according to the prior art is shown in FIG. 3. It is composed of a first operational amplifier AMPa, a second operational amplifier AMPb, first and second output resistors R1a, R1b, a feedback loop LPa for an AC signal and a feedback loop LPd for a DC voltage, wherein each of these two feedback loops is encircled in a respective chained line in FIG. 3.
The AC feedback loop LPa is composed of a differential amplifier DFA and coupling capacitors C1a and C1b. The DC feedback loop LPd is composed of the resistors Rx and Ry, a first lowpass filter LPF1 and a third operational amplifier AMPd. The reference characters Vx and V.sub.B indicate respective power supply voltages and G is ground.
The operational amplifiers AMPa, AMPb and AMPd are of the so-called voltage follower type, wherein the output terminal of each of these operational amplifiers is directly connected to a respective negative-going input terminal (not shown). Each has the characteristics of unitary voltage gain, non-conversion of polarity, extremely high input imedance and extremely low output impedance.
The wire A and the wire B indicate a two-wire subscriber line, to which DC current is supplied by the battery feed circuit. Since the operational amplifiers AMPa and AMPb have unitary voltage gain, each supplies the same output voltage as the respective input voltage V1a and V1b to the respective wires A and B via the respective resistors R1a and R1b.
The DC path for the subscriber loop is composed of the following serially connected elements:
the positive grounded terminal of a battery (not shown); PA1 the positive power supply terminal G of the operational amplifier AMPb; PA1 the operational amplifier AMPb, PA1 the resistor R1b; PA1 the connection point to the second wire B of the subscriber line; PA1 the second wire B of the subscriber line (not shown); PA1 a telephone transmitter/receiver set (not shown); PA1 the first wire A of the subscriber line (not shown); PA1 the connection point to the first wire A of the subscriber line; PA1 the first output resistor R1a; PA1 the output terminal of the AMPa; PA1 the negative power supply terminal of the AMPa; and PA1 the negative terminal V.sub.B of a battery (not shown).
The operational principle of the AC feedback loop with a differential amplifier circuit was already disclosed in U.S. Pat. No. 4,007,335.
On the wires A and B, the voltages V2a and V2b include the telephone signals corresponding to the AC signals superimposed on the respective DC voltages. The AC components of V2a, V2b are coupled to the differential amplifier DFA having unity gain through the respective capacitors C1a and C1b. The differential amplifier DFA also has unity gain and provides the following two signal outputs: EQU Va=-{(V2b-V2a)}ac/2 EQU Vb={(V2b-V2a)}ac/2
wherein {} ac denotes the AC component of the term in the curly brackets. The output signals Va, Vb are respectively supplied to the operational amplifiers AMPa and AMPb through the capacitors C2a and C2b.
The values of the resistances R2a and R2b and of the capacitances C2a and C2b are determined by requiring that: EQU 2.pi.f C2a R2a&gt;&gt;1 EQU 2.pi.f C2b R2b&gt;&gt;1
The AC components of the signals V1a and V1b supplied to the operational amplifiers AMPa and AMPb are respectively: EQU {V1a}ac=-{(V2b-V2a)}ac/2, EQU {V1b}ac={(V2b-V2a)}ac/2,
The telephone signal is thusly transmitted in balanced mode on the subscriber line in a manner for which {V2a}ac=-{V2b}ac. Thus the telephone signal can be discriminated from induced noise for which the respective voltages are {V2b}ac={V2a}ac.
The AC components of the output voltages of the operational amplifiers AMPa and AMPb are, therefore, {V2a}ac and {V2b}ac, respectively. Therefore, the AC output impedances Za(ac) and Zb(ac) of the respective operational amplifiers AMPa and AMPb as seen from wires A and B are very large, since: EQU Za(ac)={V2a/((V2a-V2a)/R1a)}ac=.infin. EQU Zb(ac)={V2b/((V2b-V2b)/R1b)}ac=.infin.
The fixed bias voltage Vx is applied to the operational amplifier AMPb at its input terminal through a high resistance resistor R2b. The value of the bias voltage Vx is determined to be larger than the sum of the saturation voltage of the operational amplifier AMPb and the AC signal voltage applied to its input terminal.
Since the operational amplifier AMPb has unitary voltage gain and has no DC voltage feedback loop, it outputs a DC voltage of the same amplitude as the input DC voltage, and it has an extremely low output DC impedance. That is, the operational amplifier AMPb can be considered as a voltage source having a constant voltage Vx and to have zero output impedance.
Since the operational amplifier AMPa is connected to the DC voltage feedback loop, the output DC voltage of the operational amplifier AMPa varies with the output voltage of the DC feedback loop LPd. The output of the DC feedback loop LPd is connected to the first wire A of the subscriber line.
The resistors Ry and Rx in the loop LPd pick up the line voltage V2a and supply the divided voltage V2a Ry/(Rx+Ry) to the lowpass filter LPF1 which eliminates the AC component from the divided voltage, namely the lowpass filter LPF1 supplies only the DC component {V2a Ry/(Rx+Ry)} dc of the divided voltage to the operational amplifier AMPd.
Since the operational amplifier AMPd is a voltage follower, it provides the same amplitude and polarity as the input DC voltage transmitted by the LPFl. The DC output voltage of the operational amplifier AMPd is supplied to the operational amplifier AMPa, which provides an output of the same voltage and polarity as the input thereto.
The output DC voltage Voa of the operational amplifier AMPa is: EQU Voa=V1a={V2a.multidot.Ry/(Rx+Ry)}dc
The DC impedance Za(dc) looking from the wire A towards the operational amplifier AMPa is given as: ##EQU1##
The DC impedance Zb(dc) looking from the wire B towards the operational amplifer AMPb is: EQU Zb(dc)=R1b
Therefore the total DC impedance Zdc of the battery feed circuit is: EQU Zdc=R1b+R1a(1+Ry/Rx)
In conclusion, the AC impedance for the telephone signal is infinite and the DC impedance for the DC voltage can be set to a specified value by choosing the values of the resistances.
The impedance of the battery feed circuit for the induction noise signal also can be low, because the induction noise signal, such as a 60 Hz power line induction, is removed by the differential amplifier DFA and the lowpass filter LPF1. The impedance for the induction noise signal as seen from the two wires A and B is R1a+R1b. The values of the resistors R1a and R1b are selected to be small values to reduce the heat loss in the resistors.
In the prior art battery feed circuit of FIG. 3, the total impedance Zdc as described above is: EQU Zdc=R1b+R1a(1+Ry/Rx)
and Zdc is usually set to 400 or 440 ohms.
In order to reduce the DC power consumption of the circuit, therefore, it is necessary to select small values of the output resistors R1a and R1b, and a large ratio for the values of resistors Rx and Ry.
The output voltage of the operational amplifier AMPb is a fixed voltage Vx, so that power consumption of the operational amplifier AMPb can be reduced by selecting a small value for the bias voltage Vx. The output voltage of the operational amplifier AMPa is given by the product of the DC current I of the subscriber line loop and R1a(Ry/Rx), wherein I is given by: EQU I=(V.sub.B -Vx)/(R.sub.L +R1b+R1a(1+Ry/Rx))
and R.sub.L is the load resistance of the subscriber line loop, which includes a resistance R1 of the two-wire subscriber line and a resistance Rt of a telephone set. The resistance Rt of a telephone set is about 100 ohms. Therefore, the load resistance R.sub.L ranges from 100 to 1900 ohms when the telephone set is in the off-hook status. Needless to say, when the telephone set is in the on-hook status, the load resistance R.sub.L is infinite and the current I is zero.
When the length of the line is very short, that is, when the resistance R1 is negligibly small, then the resistance R.sub.L is 100 ohms. On the other hand, the power supply V.sub.B is usually -48 volts, and the bias voltage Vx is set to about 3 volts, which yields a maximum for the loop current I of: EQU I.sub.max =(48-3)v/(100+400)ohm=90ma
The total power loss of the circuit for a current I.sub.max of 90ma is EQU (90.times.10.sup.-3).sup.2 .times.400=3.24w
When the sum R1a+R1b of the output resistors is set to 100 ohms, the power loss in the operational amplifier AMPa is 3.24 w.times.3/4=2.43 w. The power loss of 2.43 w in the operational amplifier AMPa is too large to make the power feed circuit by integrated circuit (IC) technology.
The electronic circuit of FIG. 3 could be made smaller in size but the power consumption of the circuit is still a problem.